1. Field of Invention
The present invention relates to a process for structuring surfaces and, more particularly, to a plasma spray process for structuring self-cleaning glass surfaces and self-cleaning glass surfaces formed according to the process.
2. Description of Related Art
Conventional glass surfaces that are exposed to rainwater require frequent cleaning to remove the deposits or residues that remain when water droplets containing dissolved or suspended solids evaporate. The cleaning of glass surfaces is time consuming and costly. A self-cleaning glass surface is highly desired.
The leaves of some plants such as the lotus plant, for example, are self-cleaning in the sense that water droplets tend to roll off the surface of the leaves instead of evaporating from the leaves. The lotus plant excretes small amounts of wax on the surface of the leaf in the form of very small bumps or nodules that are spaced apart a small distance from each other to create a micro-rough surface. The wax is very hydrophobic, which causes rainwater to form beads or droplets with a very small contact angle rather than a continuous wet film on the surface of the leaves. The water droplets roll off the surface of the lotus leaves taking dissolved and suspended solids with them. In this sense, the leaf surface of the lotus plant is self-cleaning, which is sometimes referred to as “the lotus effect”.
Efforts to duplicate the lotus effect on the surface of glass have not been particularly successful. The application of small bumps or nodules of wax to the surface of glass would be difficult to achieve and would be impractical because it would most likely deteriorate the transparency of the glass. Furthermore, because waxes are softer than glass, any mechanical contact with the glass surface would likely damage the micro-rough surface structure.
Several self-cleaning glass coatings have been developed in recent years based upon extremely hydrophobic perfluorinated polymers. Coatings formed using perfluorinated polymers are typically bonded to glass surfaces or intermediate coating layers using functional polysiloxanes. Unfortunately, such coatings do not impart super-hydrophobicity (contact angles greater than 140°) because they tend to have a relatively smooth surface. Furthermore, these coatings are not particularly durable. The properties of perfluorinated polymer coated glass surfaces are merely hydrophobic (contact angles ˜100°), and are not truly self-cleaning.
An alternative method of forming a self-cleaning glass surface is to create a micro-rough or micro-structured glass surface. Surface structures of this type feature regular or irregular peaks and valleys of 0.1 μm or greater. Depending on the surface treatment of the structured surface, the structuring can have various effects. When the surface is treated with a hydrophobic agent, the structuring tends to reduce the adhesion of water and solids and create a self-cleaning surface, called a super-hydrophobic surface. When the surface is hydrophilic, the structuring tends to aid in wetting of the surface, creating a super-hydrophilic surface.
A variety of processes and treatments have been used to produce micro-rough surfaces on glass. Examples include grit blasting, acid etching, applying and firing of glassy coatings containing non-glass particulates, sol-gel coatings and techniques that utilize chemical vapor deposition (“CVD”), combustion chemical vapor deposition (“CCVD”) and physical vapor deposition (“PVD”). These processes and treatments tend to be complicated, expensive and/or ineffective in creating durable self-cleaning surfaces.